To switch from Commercial to Gulfstreams takes about a month. You basically only need a Type Rating, which teaches you how to handle that specific aircraft, otherwise your Com licence will cover everything. It can cost up to £20,000 to get a type rating though, so its not exactly cheap. Most of the pilots I know these days prefer them though. Rich businessman buys himself a jet, pays you top money to fly with him, stay in 5 Star hotels and generally live his luxury lifestyle while being paid to do a job you'd probably do for free! In terms of backseat flying I don't really know. I never made it to Commercial level, I had to drop out for personal reasons, but I just get a warm feeling knowing that if push came to shove, I could handle myself and the aircraft in just as good a manner as the pilot.